This invention generally relates to ultrasound color flow Doppler imaging of fluid flow fields. In particular, the invention relates to a method and an apparatus for improving the display of such imaging.
Ultrasonic scanners for detecting blood flow based on the Doppler effect are well known. Such systems operate by actuating an ultrasonic transducer array to transmit ultrasonic waves into the object and receiving ultrasonic echoes backscattered from the object. In the measurement of blood flow characteristics, returning ultrasonic waves are compared to a frequency reference to determine the frequency shift imparted to the returning waves by flowing scatterers such as blood cells. This frequency, i.e., phase shift, translates into the velocity of the blood flow. The blood velocity is calculated by measuring the phase shift from firing to firing at a specific range gate.
The change or shift in backscattered frequency increases when blood flows toward the transducer and decreases when blood flows away from the transducer. Color flow images are produced by superimposing a color image of the velocity of moving material, such as blood, over a black and white anatomical B-mode image. Typically, color flow mode displays hundreds of adjacent sample volumes simultaneously, all laid over a B-mode image and color-coded to represent each sample volume's velocity.
Typically, color flow processors estimate blood flow velocity, blood flow power, and blood flow variance. Typically, color flow data is used to modify the color of a region of interest on a display screen. The user selects the type of data used for the display. The modes typically available are power only, velocity only or velocity and variance combined.
In current ultrasound scanners, various color flow display parameters are either fixed with no user selectability or are preset to some specific setting and can only be changed if action is taken by the user, one parameter at a time. This limits image quality and user productivity for any given application and scanning situation. There is a need for a scanner in which these same parameters can all be automatically adjusted at the same time to optimize image quality related to color flow display for a specific scanning situation, thus increasing user productivity.
In the color flow power or color flow velocity modes of operation, known ultrasound scanners provide user selectable color maps that are applied to the color mode data. In the power mode, the possible data values are 7-bits (0-127). In the velocity mode, the data values are signed 7-bits (-128 to +127). In a known system, any given map is fixed with the colors in the map being applied across the range of data values for power or velocity modes. There are also fixed map thresholds which are set such that color data will be displayed if it is above this threshold and will not be displayed if it is below this threshold. Such thresholding typically is used to minimize unwanted noise and artifacts. There are many scanning situations, however, where the color data in the ROI does not extend over the full possible range of output values, and therefore, many colors in the map are not used on the data which is present in the ROI. Also, the fixed map threshold may be too high for some scanning situations, causing low flow to be thresholded out even if no artifacts are present. As a result, there is a need for a color flow system in which the map threshold can be adjusted automatically.